The invention relates to a screw for osteosynthesis and arthrodesis for orthopedic surgery.
It finds a particularly important, although not exclusive, application in the field of anchors for orthopedic surgery of the hand and of the foot, more particularly at the phalanxes of the fingers and of the toes, but also of the metacarpals and metatarsals and notably for corrective surgery for the hand and for the foot.
Note than an osteosynthesis anchor must be used to keep in place two (or more) portions of one and the same bone fractured or cut by a surgical operation (osteotomy) for the time necessary for this bone to consolidate (typically three months).
An arthrodesis for its part is the immobilization of an articulation by surgical means in order to weld two bones into a single bone, by means of an osteosynthesis device.
In general the object of any osteosynthesis, and particularly in the case of an arthrodesis, is to seek a very good stability of the anchor in order to obtain the consolidation in the best conditions, that is to say in the position chosen by the surgeon, while minimizing the problems of postoperative pain and edemas, and while shortening the consolidation time as much as possible.
In order to obtain this result, the shape of the anchor is critical.
Another object sought is to provide and maintain a slight compression between the portions of bone to be fused, which makes the consolidation easier. Here again the shape of the anchor is important.
Various technical solutions have already been proposed for achieving such an osteosynthesis, notably at the extremities (foot, hand, wrist, ankle, etc.).
In addition to the conventional or shape-memory staples, it is for example routine to use screws, notably double-pitch screws allowing the placing in compression. In these screws, the pitch of each end is different (distal pitch and proximal or head pitch), for example 0.25 mm per rotation, which makes it possible to obtain a compression of 1 mm in 4 rotations. These known screws may be tubular (mounting on a pin) or not. They are usually self-tapping, that is to say that they do not usually require drilling.
Document FR 2 840 799 describes a self-tapping screw in the distal portion, for which it is nevertheless often necessary to carry out a real pre-drilling notably in the proximal portion which takes away the value of the self-drilling. Moreover, the cutting ridges of the threads are defined at the bore, which weakens the screw, that is to say that in practice the teeth thus defined break very easily.
A second frequent problem with these screws is that their length must be perfectly suited to the bony site so as not to create discomfort, and therefore that they must be screwed immediately in contact with the bone without pushing the latter away at the distal end, while ensuring a good anchorage of the end, which means a cylindrical end making good penetration impossible.
Therefore a screw is known (U.S. Pat. No. 6,306,140) comprising three portions, namely a cylindrical tapped distal portion, a smooth central portion and a self-tapping tapped proximal or head portion of larger diameter.
Such a screw has a particular angular condition only at the head, the distal portion therefore being conventional for its part, that is to say with the ridges of the screwthread being inscribed in a cylinder.
Document EP 0 856 293 A1 describes for its part a screw which on this occasion is self-tapping and self-drilling but which, in order to do this, has bevels at the ends which on the other hand detracts from the grip of the screw or makes it necessary to sink it more deeply in order that the bone is at the level at which the screw is totally cylindrical. Moreover, the proposed cutting ridges are too deep, fragile and therefore breakable at the barrel.
The invention provides a remedy for all these drawbacks in a simple, reliable, effective and rational manner.
To do this, the object of the present invention is to propose a screw that satisfies better than those formerly known the requirements of the practice notably in that it proposes a self-drilling and self-tapping, compressive screw allowing a good penetration and simultaneously ensuring a very good bone anchoring and does so without passing through the bone.
Such a screw is specially adapted to the surgical techniques that bear on the two sides of a fairly thin cortical bone as in the case of a metatarsal osteotomy of the “scarf” type.
In the rest of the text, reference will be made to the following definitions:                distal end of the screw: corresponds to the portion that first comes into contact with the bone;        proximal (or head) end: corresponds to the portion that is screwed in last and that comprises the connection to a screwing means (screwdriver for example);        barrel of the screw: corresponds to the core or generally cylindrical portion on which the screwthread is positioned.        
For this purpose, the invention therefore notably proposes an osteosynthesis and arthrodesis screw for orthopedic surgery notably of the hand and of the foot, the screw being tubular or not, having along its main axis (AP) three successive portions, namely a distal portion comprising threads (A1), a smooth central portion or central barrel (f) and a proximal portion or head comprising threads (A2), the distal portion having an external diameter that is slightly smaller than that of the proximal portion and having a screw pitch that is slightly greater than that of the proximal portion, making it possible to place in compression the two bony portions to be fused together, the bone-attack zone in the proximal portion (A2a) being conical self-tapping, characterized in that the end of the distal portion is conical,
in that the sum of the angles a1+b1 at the distal end and a2+b2 at the proximal end, defined between, on the one hand, the main axis (AP) of the screw and the external conicity of the barrel ((f1) at the distal end and (f2) at the proximal end), namely (a1) at the distal end, (a2) at the proximal end and, on the other hand, between the main axis (AP) of the screw and the crest line of the threads (P) of the screwthread, namely (b1) at the distal end and (b2) at the proximal end, is greater than 45°,
and in that the attack portion, that is to say the most distal of each thread, has a plurality of cutting ridges (AR) obtained by removal of material.
“Diameter slightly less than” means a difference of between one fifth and one twentieth of the diameter of the proximal portion and advantageously and for example a diameter of the distal portion equal to the order of nine tenths of the diameter of the proximal portion.
“Screw pitch slightly greater than” means a screw pitch of the distal portion greater than a value of between 5% and 20%, and advantageously of the order of 10%, of the screw pitch of the proximal portion.
The two threaded zones, whether they be distal or proximal, for their part make it possible to achieve the bony anchorage, each of the two threads being self-drilling and self-tapping.
The invention is based on the idea of achieving in combination two anchoring zones biting into the bone when screwed in, one for the proximal portion and the other for the distal portion, with particular external profiles, namely slightly conical (thinner on the distal side) out of which the also conical threads are cut, both of them, at the distal end and at the proximal end, comprising several cutting ridges each made by a longitudinal cut in the screwthread.
More precisely, in the embodiment that is more particularly described here, the distal portion, the body of which is of slight conicity (the angle a1 for example being between 4 and 15°), is furnished with a thread of great conicity (the angle b1 for example being between 50 and 80°) so that the profile of the crest of the threads of the end of the distal portion becomes cylindrical after the first or the second thread (see FIG. 2a).
Such an arrangement combined with ridges that are particularly cutting by reason of their formation by removal of material, allows the end to play a role of a punch which immediately catches hold and which makes it possible to place the screw without pre-drilling in a soft bone or with a slight punching in the bone in a harder bone, while having the same anchoring capacity as a screw with a purely cylindrical crest profile.
It is evident moreover that the fact that the ridges are obtained by removal of material and the fact that the profiles are specific and as described hereinabove surprisingly allow not only a good immediate penetration in the two portions of the bone, but also a good compression of the two portions of bone against one another and finally a good hold in the bone itself, and this is despite the conicity of the distal portion of the screw, and without passing through the bone.
The assembly can be anchored via the conventional manner of screws, after stabilization of the osteotomy by any appropriate instrument (tongs or forceps) and with the aid of an appropriate screwdriver, mounted on a pin (tubular screw) or not (solid), or a screw fitting making it possible to be connected to a motor.
Moreover, to take account of the anatomical characteristics, the anchoring zones are advantageously connected to the middle zone that is used for strength (notably shear strength) at the osteosynthesis focal point by more or less long connection zones.
Finally, the material constituting the anchor that is the subject of the invention allows drilling in the bone and therefore has the necessary hardness. Any anchorable material that is sufficiently hard such as stainless steel, titanium, a CrCo (chrome-cobalt) alloy, etc. is advantageously chosen in this instance.
In a particular embodiment of the invention, the anchor is therefore notably made of a titanium alloy such as the alloy known as TA6V (titanium, aluminum, vanadium alloy).
In advantageous embodiments, use is also made of one and/or the other of the following arrangements:
the cutting ridges (AR) are offset negatively relative to a radial axis of the screw, that is to say rearward relative to the direction of rotation of the screw when screwing in;
the cutting ridges (AR) are offset rearward in order to present a bone attack angle when screwing in (C) (at the distal end and at the proximal end) of between 5° and 12°;
the cutting ridges (AR) are preferably three in number;
the depth of the cutting ridges (AR) is less than or equal to the depth of the screwthread;
the depth of the cutting ridges (AR) is equal to the depth of the screwthread;
the sum of the angles (a1+b1) at the distal end and (a2+b2) at the proximal end, respectively is between 50° and 70°;
the angle (a1) at the distal end is between 4° and 10° and the angle (a2) at the proximal end is between 10° and 20°;
the cutting ridges (AR) have a relief of approximately 10° rearward in the direction of screwing in;
the screw has a flat or conical head.